Middle-aged and older Americans represent a high risk group for obesity development. In the U.S. total body fat levels rise with advancing age in men and women in the general population. The marked increase in prevalence of overweight in the last decade has been due, in part, to increases in middle-aged and older adults more than any other group. The general aim of this proposal is to determine the influence of age on the ability to adjust fuel oxidation to acute changes in diet composition. To achieve this aim, over a two year period healthy men (n=8 young; n=8 older) and women (n=8; n=8) age 18-30 years and 55-65 years will be recruited, undergo screening, food intake assessment and an initial whole room calorimetry visit to determine energy requirements. Subjects will be fed isocaloric amounts of a mixed diet, high carbohydrate diet and high fat diet in a randomized order. On the fourth day of each of diet condition, subjects will stay in the whole-room calorimeter for measurements of 24 hr energy expenditure and substrate oxidation Blood and muscle substrate levels and muscle oxidative and glycolytic enzyme activities will be assessed to provide insight into the whole-body adjustments in nutrient oxidation. A 2-week washout period will separate each condition. The results of this study may provide insight into a biological basis for the obesity development in middle-aged and older adults.